The present invention relates to an ink jet recording sheet, and particularly to an ink jet recording sheet which exhibits excellent ink absorbability, minimizes bleeding, and results in stable color density under keeping in high humidity.
In recent years, ink jet recording has increasingly resulted in enhancement of image quality which is approaching that of conventional photography. In order to achieve such conventional photographic quality, improvement has been made in the area of recording sheets. A void type recording sheet, which comprises a highly smoothened support having thereon a minute-void layer comprised of fine particles and hydrophilic polymers, exhibits high gloss, results in bright color formation, and exhibits excellent ink absorbability as well as ink drying properties. Accordingly, said recording sheet has been regarded as one of those which are most similar to conventional photographic quality. Specifically, when a non-water absorptive support is employed, it is possible to prepare high quality prints because cockling (wrinkling), which is noticed in a water absorptive support, is not present, and it is possible to maintain the highly smoothened surface.
Ink jet recording is mainly divided into two types; one in which ink, comprised of water-soluble dyes, is employed and the other in which ink, comprised of pigments, is employed. The ink comprised of pigments results in highly durable images but gloss tends to vary imagewise. As a result, it is difficult to prepare prints which exhibit conventional photographic quality. On the other hand, when an ink, comprised of water-soluble dyes, is employed, it is possible to obtain very bright and clear images of uniform gloss. Accordingly, it is possible to prepare color prints approaching conventional photographic quality.
However, ink, comprised of water-soluble dyes, results in disadvantages such as bleeding and poor water resistance due to the high hydrophilicity of said water-soluble dyes. Namely, when after printing, prints are stored at high humidity for a long period of time, or water droplets are allowed to adhere onto the print surface, said dyes tends to result in bleeding.
In order to overcome such drawbacks, it is a general practice to incorporate dye fixing materials such as cationic materials into a porous layer.
A method preferably employed is one in which, for example, an anionic dye is allowed to bond to a cationic polymer so as to be immobilized. Cited as such cationic polymers are polymers of quaternary ammonium salts, which are, for example, described in the conventional art of xe2x80x9cInk Jet Printer Gijutsu to Zairyo (Technology and Materials of Ink Jet Printers)xe2x80x9d, (published by CMC Co., Ltd., July 1998) and Japanese Patent Publication Open to Public Inspection No. 9-193532.
Further, a method has been proposed in which water-soluble polyvalent metal ions are previously incorporated into an ink jet recording sheet so that during ink jet recording, dyes are immobilized while being coagulated and fixed.
However, when such cationic polymers and water-soluble polyvalent inns are incorporated to minimize bleeding as well as to enhance water resistance, said dyes tend to coagulate on the surface. As a result, the image surface tends to result in a bronzing phenomena, in which the image surface exhibits a metallic gloss. Said bronzing tends to generally occur when prints are stored in a highly humid environment.
It has been known that compounds containing a zirconium atom and an aluminum atom are employed in ink jet recording sheets.
Japanese Patent Publication Open to Public Inspection Nos. 55-53591, 55-150396, 56-867789, 58-89391, and 58-94491 describe ink jet recording sheets comprised of water-soluble polyvalent metal salts which bond to water-soluble dyes to form minimally water-soluble salts.
Further, Japanese Patent Publication Open to Public Inspection Nos. 60-67190, 61-10484, and 61-57379 describe ink jet recording sheets comprised of cationic polymers as well as water-soluble polyvalent metal salts.
Japanese Patent Publication Open to Public Inspection No. 60-257286 describes an ink jet recording sheet comprised of basic polyhydroxyl aluminum compounds.
Further, Japanese Patent Publication Open to Public Inspection No. 10-258567 discloses a method employing a hydrophilic polymer and a water-soluble compound comprising a 4A group element, in combination, while Japanese Patent Publication Open to Public Inspection No. 10-309862 discloses a method employing a hydrophilic polymer together with a polyhydric carboxylic acid, and a zirconyl compound, also in combination.
Further, regarding compounds comprising said zirconium element, Japanese Patent Publication Open to Public Inspection No. 4-7189 discloses a method employing a porous pigment and an acid zirconium chloride compound. Said patent specification describes that by adding said acid zirconium chloride salt, desired adhesion strength is obtained employing a relatively small amount of binder, and it is possible to achieve improvement of image quality.
Japanese Patent Publication Open to Public Inspection No. 6-32046 discloses a method in which a zirconium compound is combined with silica and modified polyvinyl alcohol.
Further, European Patent No. 754,560 discloses that a water-soluble binder, a pigment, a zirconium compound, and a cationic polymer are employed in combination.
However, it was discovered that when water-soluble polyvalent metal salt, as above, were incorporated, problems occurred in which, when the added amount increased so as to obtain sufficient effects to minimize bleeding as well as to enhance water resistance, bronzing tended to occur. Further, it was also discovered that even though said basic polyaluminum hydroxide and zirconium oxychloride salts did not result in said problems, problems tended to occur in which the color of formed images tended to vary during storage after printing.
Namely, it was clarified that when an ink jet recording sheet comprising basic polyaluminum hydroxide and zirconium oxychloride in its ink absorptive layer was printed at high humidity, color variation tended to be greater compared to that printed at low humidity. As a result, immediate improvement has been demanded.
From the viewpoint of the foregoing, the present invention was achieved. An object of the present invention is to provide an ink jet recording sheet which exhibits minimized bleeding during storage after printing and improved water resistance under an application of water-soluble dyes, and exhibits excellent stability of color reproduction with respect to ambient humidity variation during printing.
Said object of the present invention was achieved employing the embodiments described below.
1. An ink jet recording sheet comprising a non-water absorptive support having thereon an ink absorptive layer comprising polyvinyl alcohol, a cationic polymer, and a compound containing a zirconium or aluminum atom other than zirconium oxide and aluminum oxide,
wherein a surface pH of said ink absorptive layer is 4 to 6 measured 30 minutes after receiving a water based ink of pH range 6 to 9 jetted from an ink jet printer in an amount of 20 ml/m2.
2. The ink jet recording sheet of item 1, wherein an average molecular weight of the cationic polymer is between 5,000 and 100,000.
3. The ink jet recording sheet of item 1, wherein the compound containing a zirconium atom is selected from the group consisting of zirconyl carbonate, ammonium zirconyl carbonate, zirconyl acetate, zirconyl nitrate, zirconium oxychloride, zirconium lactate, and zirconyl citrate.
4. The ink jet recording sheet of item 1, wherein the compound containing an aluminum atom is selected from the group consisting of aluminum chloride, basic aluminum chloride, aluminum sulfate, basic aluminum sulfate, and basic aluminum sulfate silicate.
5. The ink jet recording sheet of item 1, wherein the surface pH of said ink absorptive layer is 4.5 to 5.5 measured 30 minutes after receiving a water based ink of pH range 6 to 9 jetted from an ink jet printer in an amount of 20 ml/m2.
6. The ink jet recording sheet of item 1, wherein the ink absorptive layer is a porous layer.
7. The ink jet recording sheet of item 1, wherein said ink absorptive layer comprises boric acid or a salt thereof in an amount of 2 to 30 millimol per m2 of said ink absorptive layer, and a surface pH of said ink absorptive layer prior to receiving ink is from 3.5 to 5.5.
8. The ink jet recording sheet of item 1,
wherein said cationic polymer is represented by Formula (1), 
wherein R represents a hydrogen atom or an alkyl group; R1, R2, and R3 each represent an alkyl group or a benzyl group; J represents a bond or a divalent organic group; Xxe2x88x92 represents an anion group.
9. The ink jet recording sheet of item 1,
wherein said ink absorptive layer comprises at least two ink absorptive layers,
wherein said cationic polymer in said ink absorptive layer farthest from said non-water absorptive support comprises a repeating unit represented by Formula (1), 
wherein R represents a hydrogen atom or an alkyl group; R1, R2, and R3 each represent an alkyl group or a benzyl group; J represents a bond or a divalent organic group; Xxe2x88x92 represents an anion group.
The inventors of the present invention performed diligent investigations to overcome said problems. As a result, it was discovered that by incorporating polyvinyl alcohol, specified cationic polymers, and specified compounds comprising zirconium or aluminum atoms, it was possible to minimize bleeding and improve water resistance without the resulting bronzing. Further, it was also discovered that the layer surface pH of said ink absorptive layer was closely related to color variation due to humidity variation, and by adjusting said layer surface pH in the range of 4 to 6, color reproducibility at varied humidity, especially at high humidity was greatly improved. Based on said discoveries, the present invention was achieved.